A piloting assistance device (5) comprising a calculation unit (10) and a display unit (20). The calculation unit (10) executes stored instructions to determine at least one thrust margin for a propeller between a current thrust being exerted by said propeller and a threshold thrust corresponding to a negative power limit (Pmin), and to determine a main minimum total ground slope that can be followed by the aircraft in descent as a function of said thrust margin. Finally, the calculation unit presents a main symbol (25) on a display unit (20), the main symbol (25) representing the minimum total ground slope that can be followed by the aircraft (1) in descent, the main symbol (25) appearing superimposed on a representation (21) of the surroundings that exist in front of the aircraft (1).